This invention relates to a fluorescent display device, and more particularly to an aluminum paste used for a fluorescent display device constructed so as to permit a phosphor to emit light due to impingement of electrons thereon, a fluorescent display device including a conductive layer using such an aluminum paste and a method for manufacturing such a fluorescent display device.
Now, a conventional fluorescent display device which has been typically known in the art will be described with reference to FIGS. 6 and 7.
A conventional fluorescent display device generally designated at reference numeral 1 in FIG. 6 includes a box-like vacuum envelope 2 of which an interior is evacuated at a high vacuum and kept airtight. The vacuum envelope 2 includes an anode substrate 3 made of an insulating material, as well as a lid-like casing 6 formed of a front cover 4 made of an insulating and light-permeable material and a frame-like side plate 5 made of an insulating material.
The anode substrate 3, as shown in FIG. 7, is formed on an inner surface thereof positioned in the vacuum envelope 2 with a wiring 7 in a predetermined pattern corresponding to a display pattern 8. The wiring 7 is made of a thin aluminum film. The wiring 7 is then laminatedly formed thereon with an insulating layer 9. The insulating layer 9 is made of an insulating glass paste and deposited in the form of a thick film by printing. The insulating glass paste may be constituted of a powder of, for example, lead silicate glass, a powder of an inorganic material such as a heat-resistant pigment or the like, and a vehicle. The insulating layer 9 is formed at a portion thereof corresponding to each of segments 10 of the display pattern 8 with a through-hole 11, through which the wiring 7 is exposed. The through-holes 11 through which the wiring 7 is exposed each are filled with a conductive layer 12, which is formed of a conductive paste mainly consisting of Ag by printing.
The insulating layer 9, as shown in FIG. 7, is discretely formed thereon with an anode conductor 13 for every segment 10 of the display pattern 8 so that it may be electrically connected through the conductive layer 12 to the wiring 7. The anode conductors 13 thus discretely arranged each are provided thereon with a phosphor layer 14 so as to correspond in configuration to each of the segments 10 of the display pattern 8. The phosphor layer 14 is made of a phosphor paste consisting of a phosphor powder and a vehicle by printing. This permits an anode 15 to be provided for every segment 10 of the display pattern 8. A grid electrode 16 is arranged above the anodes 15 and filamentary cathodes 17 are then stretchedly arranged above the grid electrode 16.
In manufacturing of the conventional fluorescent display device 1 constructed as described above, an Agxe2x80x94PbO glass paste prepared by mixing an Ag powder, a glass powder and a vehicle with each other at predetermined ratios is typically used as the paste for the conductive layer 12 for filling the through-hole 11 of the insulating layer 9. The paste may have a composition of 80 to 97% by weight in Ag powder and 3 to 20% by weight in PbO glass frit.
Manufacturing of the fluorescent display device 1 is started by forming the anodes 15 defining the display pattern 8 divided into a predetermined shape on the anode substrate 3. More particularly, an thin Al film is deposited on the anode substrate 15 and then subject to patterning by photolithography, so that the wiring 7 may be formed in correspondence to the display pattern 8.
Then, the insulating layer 9 provided with the through-holes 11 is formed on the Al wiring 7 by printing and then subject to calcination at a temperature of, for example, 550 to 600xc2x0 C. Then, the through-holes 11 of the insulating layer 11 each are filled with the Agxe2x80x94PbO glass paste described above, resulting in the conductive layer 12 being formed therein. Subsequently, a graphite paste is printed on the conductive layer 12, to thereby form the anode conductors 13, which are then subject to calcination at a temperature of, for example, 550 to 600xc2x0 C. Then, the phosphor layer 14 is formed on each of the anode conductors 13 by printing and then subject to calcination at a temperature of 500xc2x0 C. or below.
Thereafter, the anode substrate 3 on which the anodes 15 are thus formed is coated on an outer periphery thereof with a low-melting glass paste, which is then subject to calcination at 500xc2x0 C. or below. Then, a mounting paste for fixing the mesh-like grid electrode 16 on the anode substrate 3 is coated on the anode substrate 3. Subsequently, the grid electrode 16 is arranged on the mounting paste, resulting in being fixed on the anode substrate 3.
Separately from the above-described operation, a frame which has the filamentary cathodes 17 stretched arranged thereon is assembled. The side plate 5 of the casing 6 is positioned at a bottom peripheral surface thereof on an outer periphery of the anode substrate 3 which is coated thereon with a low-melting paste. The anode substrate 3 and casing 6 are vertically pressed against each other and then subject to calcination at 500xc2x0 C. or below, so that the outer periphery of the anode substrate 3 and the casing 6 may be sealed to each other, so that the vacuum envelope 2 may be assembled. Finally, the envelope 2 thus formed is evacuated at a high vacuum and then sealed, so that the fluorescent display device 1 may be completed.
In manufacturing of the fluorescent display device 1, as described above, the insulating layer 9 provided with the through-holes 11 is formed on the wiring 7 made of the thin Al film by printing. Formation of only one such insulating layer 9 causes pin holes to be formed in the insulating layer 9 due to intrusion of dust or the like thereinto during printing, so that the wiring 7 and anode conductor 13 are connected to each other through the pin holes, leading to deterioration in insulation. In order to avoid such a problem, printing of the insulating paste on the wiring is carried out twice, to thereby construct the insulating layer 9 into a two-layer structure, resulting in eliminating the above-described deterioration in insulation.
Now, a conventional fluorescent display device equipped with three-dimensional grids or stereogrids will be described hereinafter with reference to FIGS. 8 and 9 by way of example, wherein FIG. 8 generally shows the fluorescent display device and FIG. 9 shows an electrode structure incorporated in the fluorescent display device. The stereogrid-equipped fluorescent display device generally designated at reference numeral 100, as shown in FIG. 8, includes a vacuum envelope 200 of a box-like shape kept airtightly and at a high vacuum. The vacuum envelope 200 includes an anode substrate 300 and a lid-like casing 600. The lid-like casing 600 is formed of an anode substrate 300 made of an insulating material, a flat plate or front cover 400 made of an insulating and light-permeable material and a frame-like side plate 500 made of an insulating material. The substrates 300, 400 and 500 of the vacuum envelope 200 each are made of glass. The anode substrate 300 is sealedly mounted on an outer periphery thereof with the casing 600 by means of a sealing substance, resulting in the vacuum envelope being provided, which is then evacuated at a high vacuum.
The anode substrate 300, as shown in FIG. 9, is formed on an inner surface thereof with a wiring 700 in a predetermined pattern. The wiring 700 is made of a thin film of a conductive material such as Al or the like. The anode substrate 300 is formed thereon with an insulating layer 900 so as to cover the wiring 700. The insulating layer 900 is formed with through-holes 800, each of which is filled with a conductive material 110 such as, for example, Ag or the like.
Also, the fluorescent display device, as shown in FIG. 9, includes a conductive material such as graphite or the like arranged in the form of a segment 101 on the conductive material 110 in each of the through-holes 800, so that the materials each constitute an anode conductor 120. The anode conductors 120 each have a phosphor layer 130 deposited thereon in the form of the segment 101.
The fluorescent display device, as shown in FIGS. 8 and 9, also includes partitions 140 each made of an insulating material and formed around each of the phosphor layers 130. The partition 140 is arranged so as to surround the phosphor layer 130 and formed into a height larger than the phosphor layer 130. Such arrangement of the partition 140 permits an anode 150 to be provided while being partitioned for every segment. The segments 101 are integrally connected to each other by means of the partitions 140 for every display pattern 160.
The partitions 140 for every display pattern 160 are led out of each of the segments 101 by a predetermined distance so as to extend outwardly of the anode substrate 300. The partitions 140 for every display pattern 160 each are formed on a top thereof with a grid electrode 190, which is made of a conductive layer by printing, resulting in constituting a stereogrid 180.
Further, the fluorescent display device, as shown in FIG. 8, includes a plurality of filamentary cathodes 170 stretchedly arranged above the display pattern 160 in the vacuum envelope 200 while being rendered opposite to the display pattern 160. The filamentary cathodes 170 each are heated to emit electrons toward the display pattern 160 while being controlled.
The stereogrid-equipped fluorescent display device thus constructed permits division of the grid electrode at a close or crowded portion of the display pattern while preventing leakage luminescence, to thereby increase a degree of freedom of the display pattern, resulting in acceleration and interruption of the electrons being effectively controlled.
However, in each of the conventional fluorescent display devices described above, the wiring 7 or 700 is covered with an aluminum oxide film during calcination of the insulating layer 9 or 900, so that the fluorescent display device fails to provide satisfactory electrical connection between the wiring 7 or 700 and the anode conductor 13 or 120 due to the aluminum oxide film. Also, repeated calcination at a temperature of 550 to 600xc2x0 C. causes separation of PbO from the Agxe2x80x94PbO glass paste, resulting in a PbO glass film being formed on the wiring 7 or 700 or the anode conductor 13 or 120, leading to a failure in satisfactory electrical connection between the wiring 7 or 700 and the anode conductor 13 or 120 through the conductive layer 12 or 110.
In order to solve the problem, the assignee proposed that a conductive paste which functions to break the oxide film on the Al wiring 7 by a chemical reaction using an activator such as Zn, Sb or the like is used as the filler paste or the paste to be filled in the through-hole, as disclosed in Japanese Patent Application Laid-Open Publication No. 29414/1995 (Japanese Patent No. 2,677,161).
The conductive paste proposed is made by mixing an Ag powder, a glass powder and a vehicle with Zn and/or Sb in an amount of 1 to 20% added as an activator. Use of the conductive paste as the filler paste permits the oxide film on the Al wiring 7 to be broken by a chemical reaction, so that the paste may act as a catalyst for promoting alloying between Al in the wiring 7 and Ag in the conductive paste.
However, use of the conductive paste containing Zn and Sb in the form of an activator as the filler paste causes the activator contained in the conductive paste to excessively exhibit a chemical reaction because calcination at 550xc2x0 C. or more is carried out several times during manufacturing of the fluorescent display device 1 shown in FIGS. 6 and 7. This results in the thin Al film being wholly corroded to decrease the conductive layer 12 in the through-hole 11, to thereby render the conductive layer 12 porous or spongy, leading to a failure in electrical connection.
Such a problem is likewise encountered with the stereogrid-equipped fluorescent display device shown in FIGS. 8 and 9 as well.
In the stereogrid-equipped fluorescent display device 100 shown in FIGS. 8 and 9, printing and drying are repeatedly carried out after formation of the insulating layer 900 by printing and calcination thereof, so that the partitions of a predetermined height and the phosphor layers 140 are formed, followed by calcination at a temperature of 550 to 600xc2x0 C. Thus, the number of times of calcination at a temperature of 550 to 600xc2x0 C. is increased as compared with that in the fluorescent display device shown in FIGS. 6 and 7, resulting in a failure in electrical connection being increased correspondingly.
In addition, the conventional conductive paste used as the filler paste mainly consists of Ag dissimilar to the wiring layer 700 made of a thin Al film, to thereby fail to exhibit satisfactory conformability to the wiring layer 700. Also, it is increased in cost as compared with the Al conductive paste.
Further, in the fluorescent display device shown in FIG. 7, the insulating layer 9 is colored black in order to increase contrast between the display pattern and a periphery thereof to enhance luminescence of the phosphor layer 14.
However, in general, a phosphor used for a fluorescent display device is nearly white. Thus, when the fluorescent display device shown in FIG. 6 is used as a vehicle-mounted display panel or a display panel for a gasoline station in an environment in which it is exposed to any external light increased in intensity such as sunlight or the like, the external light intrudes through a filter (not shown) arranged on a front surface of the front cover 4 into the vacuum envelope 2. This causes the segments 10 of the display pattern 8 which are kept turned off to be seen through due to a difference in contrast between the white phosphor layer 14 and the black insulating layer 9 and anode conductive layer 13.
Furthermore, the fluorescent display device shown in FIGS. 6 and 7 fails to restrain an increase in temperature thereof due to heat generated from the phosphor layer 14 during luminescence thereof, leading to a deterioration in luminance, because the conductive layer 13 is made of graphite increased in resistance to a level as high as 200 xcexa9/xe2x96xa1. Also, such an increase in resistance of the conductive layer 13 renders formation of the anode conductive layer 13 in a fine pattern highly difficult or troublesome.
In recent years, a fluorescent display device which has a phosphor free of cadmium (Cd) incorporated therein has been demanded in the market in view of an environmental pollution problem. Such phosphors free of Cd (hereinafter referred to as xe2x80x9cCd-less phosphorsxe2x80x9d) include, for example, Ln2O2S:Re and the like, wherein Ln is one selected from the group consisting of Y, Cd and La, and Re is Eu or Tb).
In the fluorescent display device shown in FIGS. 6 and 7, as described above, the conductive layer 13 formed by printing of a thick film is made of graphite increased in resistance as compared with Al and Ag. Also, the existing Cd-less phosphor is inherently increased in resistance. Thus, the Cd-less phosphor fails to emit light under a voltage as low as 150 V or less.
In view of the above, in the prior art, a conductive material mainly consisting of a In2O3 powder is mixed with the above-described Cd-less phosphor increased in resistance, to thereby reduce a resistance of the phosphor, leading to luminescence of the phosphor. Unfortunately, this fails to ensure uniform luminescence of the phosphor. Also, this causes the conductive material In2O3 to partially shield luminescence of the Cd-less phosphor, leading to a deterioration in luminance.
Moreover, in fabrication of the stereogrid 180 shown in FIGS. 8 and 9, an insulating paste of lead glass frit is subject to screen printing to form the partition 140, which is coated on a top thereof with a conductive paste prepared by mixing Al with lead oxide glass frit by screen printing, to thereby provide the grid electrode 190.
The lead oxide glass frit used for formation of the partition 140 may mainly consist of any one selected from the group consisting of PbOxe2x80x94B2O3, PbOxe2x80x94B2O3xe2x80x94SiO2, PbOxe2x80x94B2O3xe2x80x94ZnO, PbOxe2x80x94B2O3xe2x80x94ZnOxe2x80x94SiO2 and PbOxe2x80x94SiO2. In other words, the glass frit may contain any one selected from the group in an amount of 70% or more. Also, the glass frit may further contain a flow preventing filler such as alumina, TiO2 or the like. In addition, in the conductive paste for the grid electrode 190 which is a mixture of Al with lead glass frit, the Al powder may be in the form of a fine particle having an average particle diameter as small as about 1 to 10 xcexcm and covered thereon with an oxide film.
Then, the partition 140 and grid electrode 190 thus formed by printing are concurrently subject to calcination at a temperature of, for example, 550 to 600xc2x0 C., during which a radical combustion reaction takes place to melt the oxide film present on a surface of the conductive paste and then reduce PbO during oxidation of Al in the conductive paste, leading to precipitation of Pb. Also, the insulating paste used for formation of the partition 140 is made of lead oxide glass frit, so that precipitation of Pb is carried out at an interface between the partition 140 and the grid electrode 190 during oxidation of Al in the conductive paste as well.
The inventors carried out an experiment for confirming whether or not precipitation of Pb takes place in the conventional electrode structure. For this purpose, differential thermal analysis (TG-DTA) was carried out by subjecting the electrode structure to calcination at a rate of temperature rise of 10xc2x0 C./min within a temperature range between 30xc2x0 C. and 570xc2x0 C. using a differential thermal analysis equipment manufactured by Mac Science. The results were as shown in FIGS. 10 to 12.
FIG. 10 shows results of differential thermal analysis carried out on a fine Al powder and indicates that a vigorous oxidation reaction took place at a temperature near 540xc2x0 C. FIG. 11 shows results of differential thermal analysis on a paste within a temperature range between 30xc2x0 C. and 570xc2x0 C. The paste was obtained by mixing the fine Al powder with low-melting PbO glass frit to prepare a mixture and then calcining the mixture at a temperature of 570xc2x0 C. once. FIG. 12 indicates that a peak by endothermic appeared at a temperature near 327.3xc2x0 C. which is a melting temperature of Pb. Such results reveal that PbO was reduced during oxidation of Al in the conductive paste described above, leading to precipitation of Pb.
Such precipitation of Pb as described above causes the thus-precipitated Pb to be vaporized in the subsequent calcination step, resulting in being deposited on a surface of the filamentary cathodes 170. The calcination step includes a sealing step carried out at a temperature of 420 to 500xc2x0 C. and an exhaust step at a temperature of 300 to 350xc2x0 C. This causes Pb deposited on the surface of the filamentary cathodes 170 to be sintered to the surface when the filamentary cathodes 170 are driven for heating, leading to a deterioration in electron emission characteristics of the cathodes filamentary 170. Also, the stereogrid-equipped fluorescent display device thus deteriorated in emission characteristics causes a dark line which is a phenomenon that display just below the filamentary cathodes 170 is darkened to occur in 1000 hours, resulting in being deteriorated in life characteristics or durability of the fluorescent display device.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide an electrode structure for a fluorescent display device which is capable of exhibiting increased emission characteristics and durability.
It is another object of the present invention to provide a fluorescent display device which is capable of effectively establishing a distinction between a phosphor kept turned on and that kept turned off even in an environment in which it is irradiated with external light.
It is a further object of the present invention to provide a fluorescent display device which is capable of preventing a phosphor from being black-trimmed, leading to a deterioration in quality of display even in an environment in which external light is irradiated on the fluorescent display device.
It is still another object of the present invention to provide a fluorescent display device which is capable of restricting an increase in temperature of the fluorescent display device due to heat generated from a phosphor during luminescence thereof, to thereby ensure an increase in luminance.
It is yet another object of the present invention to provide a fluorescent display device which is capable of attaining formation of a fine pattern.
It is even another object of the present invention to provide a fluorescent display device which is capable of ensuring uniform luminescence of a phosphor under a low voltage while preventing a deterioration in emission characteristics thereof even when a phosphor increased in resistance is used.
It is a still further object of the present invention to provide a fluorescent display device which is capable of ensuring positive electrical connection between a wiring and an anode conductor.
It is a yet further object of the present invention to provide a method for manufacturing a fluorescent display device which is capable of providing a fluorescent display device attaining the above-described objects.
In accordance with one aspect of the present invention, an aluminum paste for a fluorescent display device is provided which is deposited on a glass substrate constituting a part of a vacuum envelope of the fluorescent display device, to thereby function as a conductive layer. The aluminum paste includes an aluminum powder acting as a conductive material, at least one of low-softening frit glass and organic metal acting as a fixing ingredient, and a vehicle acting as a viscous ingredient.
In a preferred embodiment of the present invention, the aluminum powder, low-softening frit glass and vehicle which are mixed with each other are 40 to 80%, 3 to 40% and 15 to 30% in amounts, respectively.
In a preferred embodiment of the present invention, the aluminum powder, low-softening frit glass and vehicle which are mixed with each other are 60 to 80%, 3 to 25% and 15 to 25% in amounts, respectively.
In a preferred embodiment of the present invention, low-softening frit glass is lead oxide (PbO) frit glass.
In a preferred embodiment of the present invention, low-softening frit glass is phosphate frit glass.
In a preferred embodiment of the present invention, low-softening frit glass is bismuth oxide (Bi2O3) frit glass.
In a preferred embodiment of the present invention, organic metal is an organic titanium (Ti) compound.
In a preferred embodiment of the present invention, aluminum powder has an average particle diameter of 1 to 10 xcexcm.
In accordance with another aspect of the present invention, a fluorescent display device is provided. The fluorescent display device includes a vacuum envelope kept at a high vacuum therein, an aluminum wiring arranged in the vacuum envelope, an insulating layer formed with through-holes and arranged so that the through-holes each are positioned on the aluminum wiring in the vacuum envelope, conductive layers filled in the through-holes in the vacuum envelope, respectively, an anode conductor made of a conductive layer and formed on each of the through-holes in the vacuum envelope, a phosphor layer deposited on an upper surface of each of the anode conductors in the vacuum envelope, and a grid arranged opposite to the phosphor layer in the vacuum envelope. Any of the conductive layers is formed by deposition and calcination of the aluminum paste prepared as described above, wherein the frit glass and/or an oxide of the metal acting as the fixing ingredient is fixed by the aluminum acting as the conductive ingredient.
In a preferred embodiment of the present invention, the insulating layer formed with the through-holes is colored black. The anode conductors formed on the insulating layer each mainly consist of aluminum and are colored white.
In a preferred embodiment of the present invention, the insulating layer formed with the through-holes is colored white. The anode conductors formed on the insulating layer each mainly consist of aluminum and are colored black.
In a preferred embodiment of the present invention, the phosphor layer is formed into a size smaller than the white anode conductor, resulting in the conductive layer white being exposed on a periphery of the phosphor layer.
Further, in accordance with this aspect of the present invention, a fluorescent display device is provided. The fluorescent display device includes stereogrids each including a partition made of an insulating material and arranged so as to surround a phosphor layer deposited on each of anode conductors and a grid section formed of a conductive layer by printing and arranged on a top of the partition. The grid section is formed by deposition and calcination of an aluminum paste containing phosphate frit glass or bismuth oxide frit glass, resulting in containing a conductive material constituted by aluminum and a fixing ingredient constituted by a metal oxide prepared by calcining the phosphate or bismuth oxide frit glass and/or organic metal.
In a preferred embodiment of the present invention, the fluorescent display device also includes an intermediate layer arranged between the partition of each of the stereogrids and the conductive layer thereof. The intermediate layer mainly consists of a metal oxide prepared by calcination of phosphate or bismuth oxide frit glass, or organic metal.
In a preferred embodiment of the present invention, the fluorescent display device also includes an intermediate layer arranged between the partition of each of the stereogrids and the conductive layer thereof. The intermediate layer mainly consists of phosphate frit glass. The intermediate layer during calcination of the stereogrid has a softening point set to be higher than a sintering temperature of the grid section and a sintering temperature set to be lower than a softening point of the partition.
In a preferred embodiment of the present invention, the insulating material for the partition mainly consists of phosphate or bismuth oxide frit glass.
In a preferred embodiment of the present invention, the conductive layer for the grid section contains phosphate or bismuth oxide frit glass identical with that the phosphate or bismuth oxide frit glass for the insulating material.
In accordance with a further aspect of the present invention, there is provided a method for manufacturing a fluorescent display device which includes stereogrids each including a partition made of an insulating material and arranged so as to surround a phosphor layer on each of anode conductors arranged in a display pattern and a grid section formed of a conductive layer on a top of the partition. The method includes the step of printing an insulating paste so as to surround each of the phosphor layers to form each of the partitions. The insulating paste is made of an insulating material mainly consisting of low-softening frit glass. The method also includes the step of printing a conductive paste on the top of each of the partitions to form each of the grid sections. The conductive paste contains a fixing ingredient constituted by at least one of a metal oxide and frit glass and a conductive material constituted by aluminum.
In a preferred embodiment of the present invention, the conductive paste is an aluminum paste containing phosphate frit glass or bismuth oxide frit glass.
In a preferred embodiment of the present invention, the partition is made of an insulating paste mainly consisting of phosphate frit glass or bismuth oxide frit glass. The grid section contains a conductive material constituted by aluminum and a fixing ingredient containing frit glass identical with the frit glass for the partition.
In a preferred embodiment of the present invention, the method further includes the step of forming an intermediate layer between the partition and the grid section.